Treatment for increasing the hydrophilicity of materials



p 1970 J. s. HAYWARD 3,526,583

TREATMENT FOR INCREASING THE HYDROPHILICITY OF MATERIALS Filed March 24,1967 RAD/0 FREQUENCY G 5' NE RA 7'01? JOHN a HAYWARL INVENTOR.

A TTORNEY United States Patent US. Cl. 204165 3 Claims ABSTRACT OF THEDISCLOSURE Method and apparatus for rendering hydrophilic the surface ofmaterials, including normally hydrophobic materials: a gas iselectronically excited to produce an activated species which is thenpassed through an ion trap, to remove charged particles therefrom, andblown across the surface to be treated.

BACKGROUND OF THE INVENTION This invention relates to the treatment ofthe surface of materials, including normally hydrophobic materials, forrendering such surfaces hydrophilic in order that they may be morereceptive to inks, adhesives, or other coatllIlgS.

Such treatment is well known, particularly in relation to polymers suchas polyethylene. In earlier prior art relating to this matter,hydrophilicity was produced by treating such surfaces with open flames,but more recently the prevalent method has been to submit the surfacesto corona discharge. However, all presently known prior art methods arenot fully satisfactory for several reasons, e.g.: heat-treating oftendeforms the material being treated; corona discharge often causespinholes in the material; electrostatic charges are induced in thematerial, raising many problems for post-treatment handling; andunusually shaped articles, such as bottles, balls, etc. cannot betreated in a reliable manner.

In some more recent attempts to improve upon the corona dischargesystems, the discharge is not directed through the material but ratherthrough a path adjacent the surface to be treated. Also, gentle streamsof air or other gases have been used to deflect the corona arc so thatit does not damage the material being treated. Nonetheless, even theseimproved prior art systems still result in occasional damage to thesurface being treated due to the action of charged particles, and thepost-treatment problems relating to charges induced in the web have notbeen overcome.

During experimentation relating to the corona discharge treatmentreferred to above, it was found that a stream of air directed across thedischarge area substantially increased the resulting area ofhydrophilicity in the direction of the air flow. Further experimentationwas undertaken to determine what effect jets of different types of gaseswould have in this expansion of the effective area of the coronatreatment, and oxygen, nitrogen and helium were all blown through andacross the corona discharge treat ment area. It was found that nitrogenand helium were more effective in producing a hydrophilic surface thanair or oxygen by many orders of magnitude, and this finding led directlyto the conception of the invention herein.

SUMMARY OF THE INVENTION The problems referred to above are avoided bythe improved method and apparatus disclosed herein which does notsubject the material to either an elevated temperature, open flames, orelectrical discharge. It has been found that normally hydrophobicsurfaces are rendered 3,526,583 Patented Sept. 1, 1970 hydrophilic whenbathed in a stream of an activated species of a gas. According to theinvention herein, a gas such as helium or nitrogen, is excitedelectronically to produce an activated species. Some ionization mayoccur during the electronic activation of the gas, and since it is oftendesirable to eliminate the possible electrostatic effects of suchionized particles, a preferred embodiment of the invention employs anion trap. In :such embodiment, the activated species is first passedthrough an ion trap to remove any charged particles and, thereafter, isdelivered in a stream to the surface being treated. This novel andsimple treatment has not only been used successfully to renderhydrophilic the surface of normally hydrophobic polymers, it has alsobeen used to increase the hydrophilicity of paper and metal.

Therefore, it is an object of this invention to provide a simple andeconomical method for rendering materials hydrophilic.

It is a further object of this invention to render materials hydrophilicwithout the use of heat, flame or corona discharge.

These and other objects, purposes and characteristic features of thepresent invention will be in part obvious from the accompanying drawing,and in part pointed out as the description of the invention progresses.In describing the invention in detail, reference will be made to theaccompanying drawing which illustrates, in schematic cross-section,simple apparatus which may be used to carry out the invention herein.

Referring now to the drawing, a gas (such as argon, helium, hydrogen,krypton, neon, nitrogen, or xenon) is released from pressurizedreservoir 1 through valve 2 to an exciter stage 3. This exciter stage iscomprised of electrode 4 and a surrounding plate 5 between which arelatively high potential radio frequency field is maintained to cause acontinuous discharge of electrons from the surface of electrode 4. Thiselectron activity causes a portion of the gas passing through thissection of the apparatus to be excited to a higher energy level, therebyforming an activated species. The gas next passes through an ion trap 6comprising oppositely charged plates 7 and 8 which attract any chargedparticles which may have been formed during the electronic excitationstage. The electricallyneutralized, activated species of the gas is thendelivered through nozzle 9 against the surface of web material 10 whichis moved past the treatment area by supply and take-up reels 11 and 12,respectively.

Special attention is called to the following important features of thenovel method and apparatus which have just been described: The web 10 isnot subjected to any great temperature increase during treatment, andtherefore web deformation is avoided. Further, since the electricalfield is remote from the surface being treated, the web is not subjectedto any electrical discharge, eliminating scouring or pinholing of thesurface being treated. Still further, if the ion trap is used the web isnot contacted by, nor does it pass in proximity to, either chargedparticles or electrodes, and therefore there is no buildup ofundesirable charges in the web, simplifying handling and storageproblems after treatment. Finally, it can readily be appreciated thatunusually shaped articles of varying thicknesses and dimensions may betreated effectively by placing them in the path of theelectricallyneutral stream of activated species emitted by nozzle 9.

To facilitate practical apparatus design, it should be noted that theeffectiveness of the active species in producing the desiredhydrophilicity declines exponentially with the distance over which it istransported, and that its effectiveness is also lost upon collision witha large mass.

The following specific examples provide further illustration of theinvention.

Example 1 The following apparatus was assembled to demonstrate that anelectrostatically neutral but chemically active species, which isgenerated in a number of different gases when they are passed through anelectrical discharge, is capable of modifying the surface of a number ofdifferent materials causing them to become more hydrophilic. Theapparatus consisted of a source of pure gas, an excitation chamber, anion trap, and a sample holder. The excitation chamber consisted of a /8"OD. stainless steel electrode supported concentrically inside a A" I.D.Pyrex tube which was wrapped on the outside with aluminum foil for alength of 3". A Tesla coil was connected to the stainless steelelectrode thus providing an R.F. field between the electrode and thegrounded aluminum foil. The ion trap consisted of two 20" long stainlesssteel plates spaced 3A6" apart by strips of Teflon. A DC. potential wasapplied between the plates which was of suflicient magnitude to removeall electrically charged species from the effluent gas. The sampleholder provided a means of supporting the surface to be treated in aplane perpendicular to the direction of gas flow and at a specificdistance from the end of the apparatus.

When nitrogen was passed through this apparatus at a flow rate of 200cc. per second a sample of polyethylene held from the exit tube (a shortlength of Pyrex tubing fastened to the ion trap) showed a significantincrease in its hydrophilicity after being treated for 10 seconds. Thetechnique of measuring the angle between the sample surface and thetangent at the point of contact of a drop of water was one method usedto detect changes in hydrophilicity.

Example 2 Example 1 was repeated using helium in the system instead ofnitrogen. The results were essentially the same.

Example 3 Example 1 was repeated using hydrogen in the system instead ofnitrogen. Again the results showed an increase in hydrophilicity.

Example 4 Example 1 was repeated using air instead of nitrogen. In thiscase the treatment time had to be increased about 70 times in order toget the same level of increase in hydrophilicity.

Example 5 Example 1 was repeated using oxygen instead of nitrogen. Theresults here were about the same as for air.

Example 6 Example 1 was repeated but in this case the sample treated wasa piece of stainless steel having a polished surface. After a treatmenttime of 30 seconds the surface was shown to be more hydrophilic.

Example 7 Example 1 was repeated using a variety of sample materialsincluding polyesters, cellulose acetate, and paper. All of the materialstested showed their surfaces to be more hydrophilic after treatment.However, the treatment time required for a given change variedconsiderably among the different samples.

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention.

I claim:

1. A method for increasing surface hydrophilicity of a normallyhydrophobic polymeric material comprising the steps of exciting a gasselected from the group consisting of argon, helium, hydrogen, krypton,neon, xenon, nitro gen, and mixtures thereof with a source of radiofrequency radiation, to form activated species thereof, removingsubstantially all charged particles formed during excitation, anddirecting said activated species substantially free of charged ions,into contact with the surface of said material to be treated at alocation remote from the source of radiation.

2. The method according to claim 1 wherein said step of removingsubstantially all charged particles includes: passing said activatedspecies through an ion trap prior to directing said stream onto thesurface to be treated.

3. An apparatus for treating the surface of a normally hydrophobicpolymeric material with an excited gas species to render the surfacemore hydrophilic comprising a housing having an entrance capable ofreceiving an excitable gas, radio frequency generation means within thehousing near said entrance and adapted to electronically excite the gasreceived through the entrance to form an active species thereof, thehousing provided with an exit through which the excited gas speciespasses, means for supporting said material in the path of said excitedgas species and wherein said housing is further provided with at leasttwo oppositely charged plates between which a gas stream can flow, suchplates being located inside said housing between said radio frequencygeneration means and the exit of said housing and thereby forming an iontrap for removing charged particles from the activated gas species canbe directed against a surface to be treated.

References Cited UNITED STATES PATENTS 3,294,583 12/1966Fedow-Fedotowsky 204-192 3,309,299 3/1967 Mantell 204- ROBERT K.MIHALEK, Primary Examiner US. Cl. X.R.

